The present invention relates to field ionization and, more particularly, to a microelectronic field ionizer structure and a method of fabricating the same.
There are two fundamentally different approaches to ionizing atoms or molecules of a gas or the like. In the first approach, bombardment or other energetic particle interaction with the molecules of the gas of interest is used to strip off electrons. This approach, often called "hard" ionization, can result in appreciable undesired particle interaction with the gas molecules, resulting in fundamental changes in the nature of the positive ions formed by the procedure.
The second approach, often referred to as "soft ionization", relies on interaction with the gas atoms to be ionized, of an electrostatic field rather than particles. The advantage of this approach is that it is less likely that the interaction between the field and the molecules or atoms to be ionized will result in nuclei changes. The paper published in the International Journal of Mass Spectroscopy, entitled "Characteristics of a Volcano Field Quadruple Mass Spectrometer" by C. A. Spindt, the inventor hereof, and W. Aberth, discloses a field ionization source which uses a volcano-like cathode to effect the desired ionization. U.S. Pat. No. 4,141,405 naming the applicant as the inventor, describes a method of fabricating a hollow volcano cathode for use in such an ionization source. Gas to be ionized is passed through the cone of the cathode. When an appropriately high voltage is applied between the cone and a nearby counterelectrode, a high electrostatic field is created at the sharp rim at the end of the hollow cone to ionize gas in its region. Thus, passage of a gas to be ionized through the cone results in the desired ionization.
The counterelectrode of such a structure typically is a separate screen having small circular openings which register with a volcano shaped cone through which the gas is passed.
Volcano field ionizers have been made relatively small in the past. For example, in operable embodiments the sharp rims of volcanos themselves have had a diameter of about 20 microns and have been centered in about 60 micron diameter circular screen openings positioned in the plane of the rims. With such a construction, there is a spacing of nominally 40 microns between the edge of the cone and the counterelectrode screen opening. Operating voltages in the range of 1000-2000 volts are required to create the electrostatic field necessary at the sharp rim of the cone to effect the desired gas ionization. It has been found that in order to prevent electrical breakdown between the counterelectrode screen and the cone at such high voltages, it is necessary that such ionizers be operated within a relatively high vacuum, e.g., 10.sup.-4 torr or better.